As a result of their favorable properties, including variable solvent power and low viscosity, supercritical fluids have been employed in a variety of selective extraction processes. Whereas a number of common gases exhibit desirably low critical temperatures (below 100.degree. C.), however, carbon dioxide is without question the most widely-used solvent in supercritical fluid science and technology. McHugh, M. A. and Krukonis, V. J., Supercritical Fluid Extraction, Butterworths, Stoneham, Mass. (1986). CO.sub.2 is readily available, inexpensive, relatively non-toxic, non-flammable, and exhibits a critical temperature of only 31.degree. C. Carbon dioxide is also one of the few organic solvents which occurs naturally in large quantities. Moreover, because CO.sub.2 is a gas under ambient conditions, reduction of liquid or supercritical CO.sub.2 -based solutions to atmospheric pressure induces essentially complete precipitation of solute, thereby facilitating solute/solvent separation.
Consequently, supercritical CO.sub.2 has been tapped as an environmentally-sound, organic solvent in such diverse areas as chromatography, biotechnology, polymerization and extraction of thermally-labile constituents from natural products. McLaren, L., et al., Science, 159, 197 (1986); Giddings, J. C., et al., Science, 162, 67 (1986); Randolph, T. W. et al., Science, 238, 387 (1988); Russell, A. J. and Beckman, E. J., Appl. Biochem Biotech., 31, 197 (1991); Desimone, J. M., et al., Science, 257, 5072 (1992); Hubert, P. and Vitzhum, O. G., Angew. Chem. Int. Ed., 17, 710 (1978).
Notwithstanding carbon dioxide's inherent advantages, however, carbon dioxide is a relatively non-polar material and thus will not solubilize highly polar, hydrophilic, or metallic solutes to a significant degree. The most commonly applied strategy for overcoming the poor solubility of polar solutes in CO.sub.2 is addition of a co-solvent (also known as a modifier or entrainer), such as a low molecular weight alcohol. Kim, S. and Johnston, K. P., AIChE J., 33, 1603 (1987). The primary disadvantage of this strategy, however, lies in the need to include a large fraction of alcohol to solvate small amounts of solute. Inclusion of large fractions of co-solvents such as alcohol increases the critical temperature of the solvent (now a CO.sub.2 -alcohol mixture), and thereby increases the required process temperature and pressure. Moreover, even addition of alcohol co-solvents will not solubilize significant quantities (mole fractions greater than 10.sup.-3) of hydrophilic solutes such as metals or proteins.
In the late 1980's, researchers at the University of Texas and Battelle's Pacific Northwest Laboratories investigated the use of commercial surfactants and chelating agents to improve the solubility of polar solutes in non-polar supercritical fluids ("SCF's"). Lemert, R. M.; et al., J. Phys. Chem., 94, 6021 (1990); Fulton, J. L. and Smith, R. D., J. Phys. Chem., 92, 2903 (1988). In that regard, studies show that formation of reverse micelles in supercritical alkanes dramatically increases the SCF solubility of amino acids, water-soluble polymers, proteins, and metal-containing compounds. Beckman, E. J., et al. Supercritical Fluid Technology, Bruno, T. J. and Ely, J. F., Eds., CRC Press, Chapter 12 (1991); Johnston, K. P., et al., Supercritical Fluid Science and Technology, Johnston, K. P. and Penninger, J. M. L., Eds., ACS Symp. Ser. No. 406 (1989).
However, extension of the use of surfactants/chelating agents to environmentally-benign CO.sub.2 has been blocked by the experimental observation that commercially available ionic amphophiles, while highly soluble in alkanes such as ethane and propane, exhibit poor to negligible solubility in carbon dioxide at moderate pressures (i.e., 10-500 bar). This observation holds regardless of whether the agent is a sulfonate, sulfate, ammonium halide, or phosphate. Consani, K. A. and Smith, R. D., J. Supercrit. Fl., 3, 51 (1990). Conventional chelating agents have shown identical trends. Tingey, J. M. et al., J. Phys. Chem., 93, 2140 (1989).
In an attempt to overcome this problem, one study evaluated improvements of the solubility of diethyl thiocarbamate-metal chelates upon replacement of the alkyl groups thereof with fluoroalkyl moieties. Laintz, K. E., et al., J. Supercrit. Fl, 4, 194 (1991). Although this strategy led to a two to three order of magnitude increase in solubility (at 50.degree. C., 1500 psi), the greatest absolute solubility (approximately 4.6.times.10.sup.-4 gm/gm CO.sub.2) achieved is too low to act as the basis for a large-scale extraction process.
Another study with fluoroalkyl-functional, twin-tailed sulfonates has shown that a fluoroalkyl chain length of C.sub.5 or C.sub.6 and greater is required to generate 1-5 weight percent solubility of sulfonates in CO.sub.2. Hoefling, T. A., et al., J. Phys. Chem. (1991).
At present, the poor solubility of conventional chelating agents in CO.sub.2 has prevented process extraction of metals using such chelating agents in CO.sub.2. Because of the advantageous properties of CO.sub.2 described above, however, it is very desirable to develop a method and chelating agents suitable for performing such extractions.